diff --git a/app/models/derived_variables/sales_log_variables.rb b/app/models/derived_variables/sales_log_variables.rb
index 16fb6ebd7..525c12936 100644
--- a/app/models/derived_variables/sales_log_variables.rb
+++ b/app/models/derived_variables/sales_log_variables.rb
@@ -21,12 +21,19 @@ module DerivedVariables::SalesLogVariables
     self.pcode1, self.pcode2 = postcode_full.split(" ") if postcode_full.present?
     self.totchild = total_child
     self.totadult = total_adult + total_elder
-    self.hhmemb = totchild + totadult
+    self.hhmemb = number_of_household_members
     self.hhtype = household_type
   end
 
 private
 
+  def number_of_household_members
+    return unless hholdcount.present? && jointpur.present?
+
+    number_of_buyers = joint_purchase? ? 2 : 1
+    hholdcount + number_of_buyers
+  end
+
   def total_elder
     ages = [age1, age2, age3, age4, age5, age6]
     ages.count { |age| age.present? && age >= 60 }
diff --git a/spec/fixtures/imports/sales_logs/discounted_ownership_sales_log.xml b/spec/fixtures/imports/sales_logs/discounted_ownership_sales_log.xml
index bc1d5f274..b697c4960 100644
--- a/spec/fixtures/imports/sales_logs/discounted_ownership_sales_log.xml
+++ b/spec/fixtures/imports/sales_logs/discounted_ownership_sales_log.xml
@@ -292,7 +292,7 @@
     <Q43CashDeposit override-field=""/>
   </Group>
   <Group>
-    <HHMEMB>0</HHMEMB>
+    <HHMEMB>1</HHMEMB>
     <TOTADULT>0</TOTADULT>
     <TOTCHILD>0</TOTCHILD>
   </Group>
diff --git a/spec/models/sales_log_spec.rb b/spec/models/sales_log_spec.rb
index 7b9f7c7ef..897303b56 100644
--- a/spec/models/sales_log_spec.rb
+++ b/spec/models/sales_log_spec.rb
@@ -283,6 +283,12 @@ RSpec.describe SalesLog, type: :model do
       expect(record_from_db["hhmemb"]).to eq(6)
     end
 
+    it "correctly derives and saves hhmemb if it's a joint purchase" do
+      sales_log.update!(jointpur: 2, jointmore: 2)
+      record_from_db = ActiveRecord::Base.connection.execute("select hhmemb from sales_logs where id=#{sales_log.id}").to_a[0]
+      expect(record_from_db["hhmemb"]).to eq(5)
+    end
+
     it "correctly derives and saves totchild" do
       record_from_db = ActiveRecord::Base.connection.execute("select totchild from sales_logs where id=#{sales_log.id}").to_a[0]
       expect(record_from_db["totchild"]).to eq(2)